It is often desirable to analyze a specific component, compound, or analyte within a biological fluid such as urine. Frequently, this involves analyzing a liquid or a particulate of the biological fluid. Centrifugation is commonly used to separate a liquid solution from a particulate suspended within the liquid solution. Once separated by centrifugation the fluid is readily available for analysis. If, however, an analyte of interest is in the deposited particulate material, a more complicated process is required. The particulate material must be re-suspended and transferred from a centrifuge tube to an analysis tube. If the particulate material must be extracted prior to analysis, one or more reagents may be introduced directly into the centrifuge tube or may be introduced to the transferred, re-suspended sample in the analysis tube. If, after extraction, the analyte must be separated from other larger particulates, the sample may have to be re-centrifuged or filtered prior to analysis.
There are a number of drawbacks to the use of centrifugation. Centrifugation equipment is costly and requires a substantial amount of space. Centrifugation is a labor burden and a time burden for the operator because the centrifuge has to be loaded and unloaded. Operator error also can occur with centrifugation. Smaller, less expensive centrifugation equipment is available, but this does not eliminate the time required to process a sample, and may increase the processing time to provide adequate separation.
Plunger-like in-container pressure filtration systems have been designed to separate particles from liquid samples being tested in an effort to eliminate the necessity of centrifugation. A number of these systems involve a tube such as a test tube and a plunger mechanism that reciprocates axially within the tube. The plunger mechanism includes a filter unit at a distal end of the plunger mechanism. Downward axial movement of the filter unit via the plunger mechanism compacts particulates in the liquid sample at the bottom of the tube. Any material greater than the pore size of the filter is trapped under the filter assembly. The liquid solution can be decanted or aspirated away.
A problem with these plunger-like devices and similar devices is that they do not allow for easy recovery of the particulate material for subsequent processing. Also, the filter unit is subject to sufficient pressure during the plunging process, which can cause the filter to crack or tear. Often such devices are used with a tapered tube. Upon downward axial movement of the plunger mechanism, the diameter of the plunger mechanism and the filter unit may become the same as the inner diameter of the tapered tube. This may prevent the filter unit from being forced through the solution. If all of the liquid has not been filtered though the device, then residual liquid may contaminate the particulate material. The volume of liquid may not be easily detectable or apparent, but the contamination can be substantial. Forcing the plunger mechanism further into the tube can cause the tube to crack or break. If the plunger mechanism is not designed for removal, additional processing such as extraction of the solid within the same device may be impossible. A more complete multiple unit or multiple module device may be required that includes a removable module designed to capture the solid.